1. Field of the Invention
The present invention relates generally to the production of asphalt and soil remediation, and in particular, to a method and apparatus for remediating and disposing of contaminated soil.
2. Discussion of the Related Art
The disposal of soils which are contaminated with petroleum products, such as kerosine, gasoline or oil is an area of increasing environmental concern. The Environmental Protection Agency (EPA) has imposed increasingly strict regulations on the clean-up and disposal of contaminated soils.
A leaking underground fuel tank is a common form of the problem. Once a defective tank is removed, all of the surrounding contaminated soil must also be removed. This can involve the removal and disposal of thousands of tons of soil that have been contaminated. The alternative of disposing the contaminated soil into landfills is no longer as attractive due to decreasing landfill space and stricter EPA and state government regulations. Therefore, treatment of the contaminated soil is an increasingly more viable solution.
A common method of decontamination is to incinerate the soil. Various methods of soil incineration are known in the art. These methods often involve common components used by conventional asphalt production plants. However, incineration of the soil is not completely effective since the exposure of the soil to high temperature tends to produce large quantities of nitrogen oxides which are released into the atmosphere. In addition, a soil decontamination facility may not be as cost effective in all instances because the process produces only remediated soil which still must be discarded. Therefore, a need exists for a cost effective and efficient process to both remediate and dispose of contaminated soils.
U.S. Pat. No. 5,176,445 discloses an apparatus that resembles a conventional asphalt production plant. This apparatus can be retrofitted by the addition of a small number of additional components so as to permit the apparatus to selectively produce asphalt, or to decontaminate soil. However, this apparatus suffers from marked disadvantages. First, asphalt production and soil remediation cannot occur simultaneously. Only one process can be conducted at a given time. Second, after soil remediation, the decontaminated soil must still be discarded.
U.S. Pat. No. 5,129,334 discloses an apparatus for heating and drying stone aggregate useful in the production of asphalt, and which also has the ability to add incinerated soil directly into the aggregate. While this apparatus achieves an objective of disposing of the decontaminated soil into the asphalt, it suffers from several disadvantages.
For instance, the disclosed apparatus utilizes both a rotary drum to heat and dry the aggregate and a separate rotary incinerator for decontaminating soil. The use of two separate mechanisms increases the cost of operation and maintenance of an asphalt plant. In addition, the disclosed apparatus remediates soil by exposure to high temperature within the rotary incinerator, which produces high levels of pollutants which are released into the atmosphere.
Still other systems have been proposed to produce asphalt and remediate soil using a single drum mixer/dryer. Such systems subject contaminated soil to the heat from the burner flame before the flame enters the body of the mixer, thereby heating the soil. The soil and aggregate are then combined as they are discharged together from the mixer. Such systems are relatively inefficient because the heat of remediation comes not from the aggregate but from the flame. Imperfect mixing may also result because the aggregate and soil are combined for at most a brief period of time before being discharged from the mixer.
U.S. Pat. No. 4,867,572 discloses a fixed sleeve rotary drum mixer used in the production of asphalt. A drum mixer of this type contains a fixed sleeve that surrounds the lower portion of a rotating drum. Heated and dried aggregate that passes through the drum is then discharged into an annular chamber which is formed between the drum and sleeve. In some designs, an inlet is provided in the sleeve by which recycled asphalt pavement (RAP) may be introduced into the annular chamber. Another inlet may also be provided in the sleeve to introduce liquid asphalt. Mixing blades mounted on the drum within the annular chamber mix the materials and cause them to be moved towards a discharge outlet of the sleeve.
An advantage of a fixed sleeve rotary drum over other continuous-mix rotary drums involves the ability to introduce RAP directly into the annular chamber between the drum and sleeve. The RAP is heated by contact with the hot aggregate, by conductive heat transfer from the mixing blades mounted on the drum, and by radiant heat transfer from the shell of the drum within the annular chamber. This process sufficiently heats the asphalt contained in the RAP, yet shields the RAP from exposure to the high temperatures present within the drum, which can cause the RAP to smoke and burn. This process will also equally prevent the smoking and burning of liquid asphalt, which may be introduced into the annular chamber.
A further advantage of a fixed sleeve rotary drum is the ability to provide an apparatus for the production of asphalt which is inexpensive to manufacture and operate, and provides high thermal efficiency. However, while this apparatus achieves many advantages over others seen in the art, it suffers the distinct disadvantage of being unable to remediate soil.